Fix PR 65177: diamonds are not valid execution threads for jump threading

[Sebastian Pop]

Jeff Law wrote:
> >	PR tree-optimization/65177
> >	* tree-ssa-threadupdate.c (verify_seme): Renamed verify_jump_thread.
> >	(bb_in_bbs): New.
> >	(duplicate_seme_region): Renamed duplicate_thread_path.  Redirect all
> >	edges not adjacent on the path to the original code.
> OK for the trunk.  

Committed r221675.

> Though I think there's some stage1 refactoring that we're going to want to do.

Agreed.

> Specifically, it seems to me that copy_bbs should be refactored into
> copy_bbs and copy_bbs_for_threading or somesuch.  Where those
> routines call into refactored common subroutines, but obviously
> handle wiring up the outgoing edges from the copied blocks
> differently.
> 

That would be a good cleanup: I don't like to arbitrarily redirect edges in
copy_bbs just to redirect them back to their initial place in the caller.

> The goal would be to eliminate the overly complex block copy/CFG
> update scheme in tree-ssa-threadupdate.c as part of a larger project
> to convert to a backward threader that can run independently of DOM.

I have a start of a patch for that cleanup, it currently runs wild as it would
replace the existing threadupdate code generator with a call to the new
duplicate_thread_path.  I think we should take smaller more manageable steps to
ease the review and to not destabilize the jump-threader.  In particular I think
we should have both code generators for a while and turn one on/off with an option.

Sebastian